Flat knee boots with a bit of space around the calf

Help! I would really like a pair of knee high boots, smart enough for work, need to be flat (I hate heels). I have a great pair, but I am pregnant and in my last pregnancy I couldn't zip them up after a bit. I'm actually quite slim, but I have muscly calves, and even though I didn't really put much weight on in pregnancy I still couldn't do them up. And I want a pair that in future I can wear over jeans (assuming not horrendously out of style. ). So the plan is that I will be able to do this when normal sized and able to wear while pregnant. Am willing to spend a bit as I find I wear mine all the time in winter.